BO 2727 (a.k.a. L 739,428) of the formula [1R,5R, 6S)-6-[(R)-1-hydroxyethyl]-2-{(3S,5S)-5-[(R)-1-hydroxy-3-N-methylaminoprop yl]pyrrolidin-3-yl-thio}-1-methyl-1-carbapen-2-em-3-carboxylic acid is a new .beta.-methyl-carbapenem that as proven to be a very effective broad spectrum antibiotic (Nakagawa et al., 1993). The synthesis of BO 2727 is a challenging one, because the molecule has a total of seven chiral centers. The mercaptopyrrolidine side chain of BO 2727 is derived from (R)-.beta.-hydroxyester, which has three chiral centers. This (R)-.beta.-hydroxyester is chemically synthesized from the .beta.-ketoester precursor by chiral reduction.
As an alternative to chemical synthesis of the L-739,428, a bioconversion process employing Mortierella alpina strain MF5534 is presented. In this bioconversion process the .beta.-ketoester is converted to the (R)-.beta.-hydroxy ester. This bioconversion process yields to rapid process improvements and optimization of bioconversion conditions.